The high frequency circuit section of a cell phone, smart phone, or other portable terminals includes a transmitting circuit, a receiving circuit, and a high frequency switching circuit. Generally, the transmitting circuit and the receiving circuit are connected to a shared antenna via the high frequency switching circuit. Currently, most portable terminals are made to be multi-mode and multi-band compatible. As a result, the number of ports needed for the high frequency switching circuit has increased to accommodate multi-mode/multi-band operations. As the port number increases, the number of bits of the control signal needed for controlling the connecting state of the high frequency switching circuit also increases.
For an integrated circuit (IC) containing the high frequency switching circuit, in order to decrease the number of terminals needed for the control signals, serial control signals may be input. In this case, to convert the serial control signal to a parallel control signal, a series-parallel converter is arranged inside the IC.
The series-parallel converter carries out the series/parallel conversion for the control signal in synchronization with a clock signal. The frequency of the clock signal can be about 26 MHz, for example. In this case, the rise time and fall time of the clock signal each are about 1 ns and a harmonic of about 1 GHz is generated. Because this harmonic has a frequency close to the frequency of the high frequency signal switched by the high frequency switching circuit, the harmonic will be superposed as noise on the high frequency signal.